Take the present price over the purchase price raised to the power of the reciprocal of the annualized period and then subtract one. This will get you the compounded annualized return rate.

Hopefully the example below will better explain.

Purchase SBUX at $21.00 at 1/1/03
Present Price is 58.58 at 12/3/04
Annualized Period is 1.92 years

The Formula would be

(58.58/21.00)^(1/1.92)-1 = 70.6%

or

Purchase SBUX at $47.00 at 10/3/03
Present Price is 58.58 at 12/3/04
Annualized Period is .167 years

The Formula would be

(58.58/47.00)^(1/.167)-1 = 274%